NETLINK(4) Linux Programmer's Manual NETLINK(4)
NAME
netlink, PF_NETLINK - Communication between kernel and
user.
SYNOPSIS#include<asm/types.h>#include<sys/socket.h>#include<linux/netlink.h>netlink_socket=socket(PF_NETLINK,socket_type,netlink_family);intNLMSG_ALIGN(size_tlen);intNLMSG_LENGTH(size_tlen);intNLMSG_SPACE(size_tlen);void*NLMSG_DATA(structnlmsghdr*);structnlmsghdr*NLMSG_NEXT(structnlmsghdr*,intlength);intNLMSG_OK(structnlmsghdr*,intlength);intNLMSG_PAYLOAD(structnlmsghdr*,intlength);DESCRIPTION
Netlink is used to transfer information between kernel and
user space modules. It consists of a standard sockets
based interface for user processes and an internal kernel
API for kernel modules. The internal kernel interface is
not documented in this man page.
Netlink is a datagram oriented service. Valid sockettypes
are SOCK_RAW and SOCK_DGRAM.netlink_family selects the kernel module or netlink group
to communicate with. Currently assigned netlink families
are: NETLINK_ROUTE to receive routing updates and to mod-
ify the IP routing table. NETLINK_FIREWALL to receive
packets sent by the IPv4 firewall code. NETLINK_ARPD to
manage the arp table in user space. NETLINK_ROUTE6 to
receive and send IPv6 routing table updates.
NETLINK_IP6_FW to receive packets that failed the IPv6
firewall checks (currently not implemented). NETLINK_TAP-BASE...NETLINK_TAPBASE+15 are the instances of the
ethertap device. Ethertap is a pseudo network tunnel
device that allows to simulate an ethernet driver from
user space. Reserved are NETLINK_SKIP for ENskip and
NETLINK_USERSOCK for future user space protocols.
Netlink messages consist of a byte stream with one or mul-
tiple nlmsghdr headers and associated payload. For multi-
part messages the first and all following headers have the
NLM_F_MULTI flag set, except for the last header which has
the type NLMSG_DONE. The byte stream should be only
accessed with the standard NLMSG_* macros.
Netlink is pseudo reliable. This means it tries its best
to deliver a message to its destination(s), but may drop
messages when an out of memory condition or other error
occurs. For reliable transfer the sender can request an
acknowledgement from the receiver by setting the NLM_F_ACK
flag. An acknowledgment is a NLMSG_ERROR packet with the
error field set to 0. The user has to handle generating
acks for received messages himself. The kernel tries to
send an NLMSG_ERROR message for every failed packet.
struct nlmsghdr
{
__u32 nlmsg_len; /* Length of message including header */
__u16 nlmsg_type; /* Message content */
__u16 nlmsg_flags; /* Additional flags */
__u32 nlmsg_seq; /* Sequence number */
__u32 nlmsg_pid; /* Sending process PID */
};
struct nlmsgerr
{
int error; /* error: negative errno or 0 for acks. */
struct nlmsghdr msg; /* message header that caused the error */
};
After each nlmsghdr the payload follows. nlmsg_type can
be one of the standard message types: NLMSG_NOOP message
is to be ignored, NLMSG_ERROR the message signals an error
and the payload contains a nlmsgerr structure, NLMSG_DONE
message terminates a multipart message,
A netlink family usually has more specified message types,
see the appropiate man pages for that, e.g. rtnetlink(4)
for NETLINK_ROUTE.
Standard Flag bits in nlmsg_flags
NLM_F_REQUEST set on all request messages
NLM_F_MULTI the message is part of a multipart mes-
sage terminated by NLMSG_DONE
NLM_F_ACK reply with an acknowledgment on success
NLM_F_ECHO echo this request
Additional flag bits for GET requests
NLM_F_ROOT Return the complete table instead of a single entry.
NLM_F_MATCH Not implemented yet.
NLM_F_ATOMIC Return an atomic snapshot of the table. Requires CAP_NET_ADMIN or super user rights.
NLM_F_DUMP not documented yet.
Additional flag bits for NEW requests
NLM_F_REPLACE Override existing object.
NLM_F_EXCL Don't replace if the object already exists.
NLM_F_CREATE Create object if it doesn't already exist.
NLM_F_APPEND Add to the end of the object list.
ADDRESSFORMATS
The sockaddr_nl structure describes a netlink client in
user space or in the kernel. A sockaddr_nl can be either
unicast (only send to one peer) or send to netlink groups
(nl_groups not equal 0).
struct sockaddr_nl
{
sa_family_t nl_family; /* AF_NETLINK */
unsigned short nl_pad; /* zero */
pid_t nl_pid; /* process pid */
__u32 nl_groups; /* multicast groups mask */
};
nl_pid is the pid of the user space netlink, or 0 if the
destination is in the kernel. nl_groups is a bitmask with
every bit representing a netlink group number.
NETLINKMACROS
Netlink defines several standard macros to access or cre-
ate a netlink datagram. They are similar in spirit to the
macros defined in cmsg(3) for auxilliary data. The buffer
passed to and from a netlink socket should be only
accessed using these macros.
NLMSG_ALIGN
Round the length of a netlink message up to align
it properly.
NLMSG_LENGTH
Gets the payload length as argument and returns
the aligned length to store in the nlmsg_len field
of the nlmsghdr.
NLMSG_SPACE
Return the number of bytes a netlink message with
payload of the passed length would occupy.
NLMSG_DATA
Return a pointer to the payload associated with
the passed nlmsghdr
NLMSG_NEXT
Get the next nlmsghdr in a multipart message. The
caller must check if the current nlmsghdr didn't
have the NLMSG_DONE set - this function doesn't
return NULL on end. The length parameter is an
lvalue containing the remaining length of the mes-
sage buffer. This macro decrements it by the
length of the message header.
NLMSG_OK
Return true if the netlink message is not trun-
cated and ok to parse.
NLMSG_PAYLOAD
Return the length of the payload associated with
the nlmsghdrBUGS
This man page is not complete.
SEEALSOcmsg(3), rtnetlink(4)
VERSIONS
The socket interface to netlink is a new feature of Linux
2.2
Linux 2.0 supported a more primitive device based netlink
interface (which is still available as a compatibility
option). This old interface is not described here.
Linux Man Page 23 Oct 1998 1